1. You didn't mention that the restaurant was a national chain, so my comments are clearly a little off, but the thinking behind my reasoning still stands.
2. Notice the difference between the birthday freebies and the original scenario is quantity. In the new example, a lot of places are willing to give you a little something on your birthday, to entice you to sample their wares, in hopes you'll return on your own dime. Quid pro quo. The original example was more you being exploitive, IMO. More importantly, it's not likely to be repeatable and thus less likely to be a useful strategy for others to benefit from.
The point of your post was to help others, wasn't it?
Feel free to post as many other strategies as you wish, but remember that some people care about treating others fairly in life. Just because a person is racing to FI does not make it 'mustachian" for them to cheat others. NOT saying you did, but the ground around your gift card strategy felt a little spongy, which is why I called it out.